1. Field of the Invention
The present invention relates to an OFDM receiver and an OFDM signal receiving method for receiving an orthogonal frequency division multiplexing (OFDM) signal and demodulating the OFDM signal.
2. Description of the Related Art
A modulation system called an orthogonal frequency division multiplexing (OFDM) system is used as a modulation and demodulation system of a terrestrial digital broadcasting system. This OFDM system is a system for providing a large number of orthogonal sub-carriers in a transmission band, allocating data to amplitudes and phases of the respective sub-carriers, and digitally modulating a signal according to PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).
The OFDM system has a characteristic that, since the transmission band is divided by the large number of sub-carriers, although a band per one sub-carrier is narrowed and modulation speed is reduced, transmission speed as a whole is the same as that in the modulation system in the past. The OFDM system also has a characteristic that, since the large number of sub-carriers are transmitted in parallel, symbol speed is reduced. Therefore, in the OFDM system, a time length of a multi-path relative to a time length of a symbol can be reduced and transmission is less susceptible to a multi-path interference. Further, the OFDM system has a characteristic that, since data is allocated to the plural sub-carriers, a transmission and reception circuit can be formed by using, during modulation, an IFFT (Inverse Fast Fourier Transform) arithmetic circuit that performs inverse Fourier transform and using, during demodulation, an FFT (Fast Fourier Transform) arithmetic circuit that performs Fourier transform.
Since the OFDM system has the characteristics described above, the OFDM system is often applied to the terrestrial digital broadcast that is intensely affected by the multi-path interference. As the terrestrial digital broadcast employing such an OFDM system, there are standards such as DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and ISDB-TSB (Integrated Services Digital Broadcasting-Terrestrial Sound Broadcasting) (see, for example, “Receiver for Terrestrial Digital Sound Broadcast-Standard (Desirable Specifications) ARIB STD-B30 version 1.1”, Association of Radio Industries and Businesses, decided on May 31, 2001 and revised on Mar. 28, 2002 and “Transmission System for Terrestrial Digital Sound Broadcast ARIB STD-B29 version 1.1”, Association of Radio Industries and Businesses, decided on May 31, 2001 and revised on Mar. 28, 2002).
A transmission signal in the OFDM system is transmitted by a unit of a symbol called an OFDM symbol. This OFDM symbol includes an effective symbol that is a signal period in which IFFT is performed during transmission and a guard interval in which a waveform of a part of the latter half of this effective symbol is directly copied. This guard interval is provided in the former half of the OFDM symbol. In the OFDM system, such a guard interval is provided to improve multi-path resistance. Plural OFDM symbols are collected to form one OFDM transmission frame. For example, in the ISDB-T standard, one OFDM transmission frames are formed by two hundred four OFDM symbols. Insertion positions of pilot signals are set with this unit of OFDM transmission frames as a reference.
In the OFDM system in which the modulation of a QAM system is used as a modulation system for each of the sub-carriers, characteristics of the amplitude and the phase are different for each of the sub-carriers because of the influence of the multi-path and the like during transmission. Therefore, on a reception side, it is necessary to equalize a reception signal to make the amplitude and the phase for each of the sub-carriers equal. In the OFDM system, on a transmission side, pilot signals of a predetermined amplitude and a predetermined phase are discretely inserted in a transmission symbol in a transmission signal. On the reception side, a frequency characteristic of a channel is calculated using the amplitude and the phase of the pilot signals and a reception signal is equalized according to the calculated characteristic of the channel.
The pilot signals used for calculating a channel characteristic are referred to as scattered pilot (SP) signals.
A structure of a basic OFDM receiver of ISDB-T, which is the Japanese digital terrestrial broadcast standard, is shown in a block diagram in FIG. 17.
The OFDM receiver 100 includes an antenna 101, a tuner 102, a band-pass filter (BPF) 103, an A/D converter 104, a digital orthogonal demodulator 105, an FFT arithmetic circuit 106, a pilot-use channel estimator 107, a channel distortion compensator 108, an error correction circuit 109, a transmission-parameter decoder 110, a delay profile estimator 111, and a window regenerator 112.
A broadcast wave of a digital broadcast transmitted from a broadcasting station is received by the antenna 101 of the OFDM receiver 100 and supplied to the tuner 102 as an RF signal.
The tuner 102 includes a local oscillator 102b and a multiplication circuit 102a. The tuner 102 frequency-converts the RF signal received by the antenna 101 into an IF signal. The IF signal obtained by the tuner 102 is filtered by the band-pass filter (BPF) 103 and, then, digitized by the A/D converter 104 and supplied to the digital orthogonal demodulator 105.
The digital orthogonal demodulator 105 orthogonally demodulates the digitized IF signal using a carrier signal of a predetermined frequency (a carrier frequency) and outputs an OFDM signal of a baseband. The OFDM signal of the baseband outputted from the digital orthogonal demodulator 105 is a signal of a so-called time domain before being subjected to an FFT operation. Therefore, a baseband signal after the digital orthogonal demodulation and before being subjected to the FFT operation is hereinafter referred to as an OFDM time domain signal. As a result of orthogonal demodulation, this OFDM time domain signal changes to a complex signal including a real axis component (an I channel signal) and an imaginary axis component (a Q channel signal). The OFDM time domain signal outputted by the digital orthogonal demodulator 105 is supplied to the FFT arithmetic circuit 106, the window regenerator 112, and the delay profile estimator 111.
The FFT arithmetic circuit 106 applies the FFT operation to the OFDM time domain signal, extracts data orthogonally modulated in each of sub-carriers, and outputs the data. A signal outputted from the FFT arithmetic circuit 106 is a signal of a so-called frequency domain after being subjected to the FFT operation. Therefore, the signal after the FFT operation is referred to as an OFDM frequency domain signal.
The FFT arithmetic circuit 106 extracts a signal in a range of an effective symbol length from one OFDM symbol, i.e., excludes a range of a guard interval from one OFDM symbol, and applies the FFT operation to the extracted OFDM time domain signal. Specifically, a position where the arithmetic operation is started is any position from a boundary of the OFDM symbol to an end position of the guard interval. This arithmetic operation range is referred to as an FFT window.
In the OFDM receiver 100, the designation of this FFT window position is performed by the window regenerator 112. As the window regenerator 112, there are known, for example, means for performing window regeneration according to detection of a correlation value of a guard interval period using the OFDM time domain signal and means for estimating a delay profile of a channel using the delay profile estimator 111 described later and performing window regeneration.
The OFDM frequency domain signal obtained by the FFT arithmetic circuit 106 is supplied to an SP-signal extraction circuit 107a. The SP-signal extraction circuit 107a extracts only inserted SP signals and removes a modulation component of the pilot signals to calculate a channel characteristic in SP positions.
The channel characteristic in the SP positions calculated by the SP-signal extraction circuit 107a is supplied to a time-direction-channel estimator 107b. The time-direction-channel estimator 170b estimates, for each of OFDM symbols, a channel characteristic of a sub-carrier in which the SP signals are arranged. The time-direction-channel estimator 107b can estimate, for all the OFDM symbols, channel characteristics for every three sub-carriers in a frequency direction.
A frequency-direction-channel estimator 108b applies processing in the frequency direction to the channel characteristics calculated for every three sub-carriers by the time-direction-channel estimator 107b and calculates channel characteristics of all sub-carriers in the OFDM symbols.
As a result, it is possible to estimate channel characteristics for all the sub-carriers of the OFDM signal. A compensator 108a removes distortion due to the channel from the OFDM frequency domain signal calculated by the FFT arithmetic circuit 106 using the channel characteristics of all the sub-carriers supplied from the frequency-direction-channel estimator 108b. 
The transmission parameter decoder 110 extracts transmission parameter information from the OFDM frequency domain signal by decoding a sub-carrier in which the transmission parameter information is inserted and supplies the transmission parameter information to the error correction circuit 109.
The error correction circuit 109 applies, in accordance with the transmission parameter information supplied from the transmission parameter decoder 110, de-interleave processing to the OFDM frequency domain signal, from which the channel distortion is removed by the channel-distortion compensator 108. The error correction circuit 109 outputs the OFDM frequency domain signal as decoded data through depuncture, Viterbi, diffused signal removal, and RS decoding.
The delay profile estimator 111 calculates an impulse response of the channel and supplies the impulse response to the window regenerator 112. As a method of delay profile estimation, there are known, for example, a method of using a matched filter that sets a guard interval period as a tap coefficient using the OFDM time domain signal and a method of calculating a delay profile by subjecting a channel characteristic supplied from the time-direction-channel estimator 107b to IFFT.